The present invention relates to a method of determining the position of an electron beam in a colour selection electrode display tube. Such a method has particular, but not exclusive, application in setting-up convergence of the three electron beams in a colour selection electrode display tube. By convergence is meant that the images produced by the three electron beams should coincide at the cathodoluminescent screen normally provided on the inside of the optically transparent display window. The ever present demand for improved colour display on cathode ray tubes, especially in high definition tubes used for datagraphic displays requires ever more critical setting-up of convergence. Simultaneously the task has to be completed in a short time in order for it to be acceptable economically.
The problem in determining the convergence errors is that the apertures in the colour selection electrode only provide a sampled view of the image on the screen.
A known technique for measuring static and dynamic convergence errors is described in U.S. Pat. No. 4,441,120, issued Apr. 3, 1984. This known technique requires the use of a detector comprising at least one photosensitive cell which is composed of at least two separate photosensitive surface parts which are situated symmetrically relative to the centre of the cell. The photosensitivity of said photosensitive surface parts increases substantially linearly from the centre of the cell. In use, optical elements project light from a local area of the display screen on to the photosensitive surface parts of the photosensitive cell. Means are provided for establishing the difference in the amount of light on the photosensitive surface parts. These differences are used to set-up the correct static convergence of the display tube by means of magnetising at least one permanently magnetisable ring carried by the electron gun as a multipole. This known technique is accurate and provides satisfactory results. However the measurements and adjustments require a relatively long time to complete and in order to speed-up the manufacture of television display tubes it is desired to reduce the time taken to implement the various stages of tube manufacture. With this overall objective in mind the time required for convergence adjustments should be reduced.